A second class of time-delayed optical reservoir computers has been investigated in recent years, in which the nonlinearity comes not from a discrete element in the loop but from the nonlinear response of a laser when a delayed signal re-enters its cavity. Light input and output with the same aperture is achieved by polarization rotation with a circulator component or another suitable mechanism. In the first reports [1]}, [2]}, good results were experimentally obtained with a semiconductor laser for spoken digit recognition and chaotic time series prediction tasks. In another study that is related but not strictly categorized as a reservoir as there was no feedback component, a similar system based on a nonlinear resonant cavity was used to demonstrate optical vector-matrix product operations [3]}.
